In recent, the demands on portable electronic products such as notebooks, video cameras and cellular phones are rapidly increased, and the development of electric vehicles, energy storage batteries, robots and satellites is under active progress. Accordingly, high-performance secondary batteries capable of being repeatedly charged are vigorously researched.
Currently, nickel-cadmium batteries, nickel-hydrogen batteries, nickel-zinc batteries and lithium secondary batteries are commercially available as secondary batteries. Among them, lithium secondary batteries are spotlighted since they may be freely charged or discharged since memory effects are scarcely caused in comparison to nickel-based secondary batteries. In addition, lithium secondary batteries exhibit very low self-discharge rate and high energy density.
A battery used for portable electronic products such as notebooks should display a remaining capacity since it has a limited use time. A remaining capacity of a battery is generally displayed as a percentage based on the full charge capacity (FCC). Thus, in order to display a remaining capacity of a battery, it is necessary to exactly check and update the full charge capacity of the battery. The full charge capacity of a battery is set to a certain value when the battery is produced in a factory, but the full charge capacity is gradually decreased as the battery is repeatedly charged and discharged. Thus, the full charge capacity of a battery should be constantly updated, and there are proposed various kinds of algorithms used for updating such a full charge capacity. The full charge capacity of a battery may be calculated as a capacity actually charged to a secondary battery from a fully discharged time to a fully charged time. On the contrary, the full charge capacity may also be calculated as a capacity actually discharged from a secondary battery from a fully charged time to a fully discharged time. Users generally charge a battery again before the battery is fully discharged, so the later method is recently more frequently used.
Recently, the most important social issue in relation to batteries is the safety. In particular, notebooks and cellular phones are used more and more, and if a battery is exploded, a portable electronic product having the battery may be broken, which may cause even a fire. Thus, it is urgent to ensure the safety of a battery. Until now, various kinds of protecting devices have been used to ensure the safety of a battery. Such a protecting device intercepts a charging/discharging current by melting and cutting a fuse if an abnormal state was detected in a battery. However, seeing that explosion accidents of batteries still occur unceasingly, it is considered that the safety of a battery does not reach a satisfactory level. In addition, though a conventional battery protecting device prevents a fire caused by battery-related accidents, the battery may get scorched due to heat, which also damages electronic products.